Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $37.88   7.659%   $40.7812   $40.78   $40.80   $40.80  
  Purchase #2 $144.87   10.312%   $159.809   $159.81   $159.80   $159.80  
  Purchase #3 $7.87   5.683%   $8.3173   $8.32   $8.30   $8.30  
  Purchase #4 $240.90   7.772%   $259.6227   $259.62   $259.60   $259.60  
  Purchase #5 $5.87   6.643%   $6.2599   $6.26   $6.25   $6.30  
  Purchase #6 $50.91   12.686%   $57.3684   $57.37   $57.35   $57.40  
  Purchase #7 $46.81   6.360%   $49.7871   $49.79   $49.80   $49.80  
  7 purchase totals = $535.11       $581.9456   $581.95   $581.90   $582.00